In the field of municipal and industrial wastewater disposal it is necessary to treat the resulting wastewater sludge by heating to reduce the sludge volume by reducing its water content by evaporation and to reduce the sludge borne pathogens and its potential for vector attraction. Thermal drying of these sludges has emerged as one of the preferred treatment methods. Thermal drying typically constitutes passing sludge with a high water content through a dryer by means of a conveyor where heat may be applied to the sludge along the conveyor to increase the sludge temperature so as to reduce the water content to a desired level.
Thermal drying takes place by transferring heat energy to the sludge to elevate the sludge temperature and evaporate the water. The transfer of heat energy may be accomplished by either conduction, convection or radiation. Of the three, radiation produced by infrared heaters is the most energy efficient as it results in lower heat losses during the transfer and, when compared to forced air convection heating, a substantially smaller air emission control system.
One of the problems associated with thermal drying of organic sludges is that during drying, as the water content of the sludge decreases and the sludge solids content increases, and if energy is transferred to the sludge at the same rate, sludge combustion is likely to occur. This autogenous stage for sludge combustion typically takes place when the solids content of the sludge reaches 35 percent or above.
Sludge combustion during thermal drying is a widespread safety and operational problem which must be resolved. Generally, combustion is controlled by reducing the temperature applied to the sludge within the dryer. However, a reduction in dryer temperature results in a slower sludge drying time and a corresponding reduction in drying efficiency. A problem heretofore associated with sludge drying has been the need to balance the risk of sludge combustion during drying with the need to provide energy efficient and economical sludge drying.
Further, since sludge is typically dried in sludge dryers that are comprised of metal components, another problem associated with thermal drying is overheating the dryer's metal components. While dryers are typically designed to compensate for certain levels of heat expansion and for decreased metal strength due to elevated temperatures, if the design temperatures are exceeded by a substantial amount or for a prolonged period of time, excessive expansion and metal fatigue may cause permanent damage to the sludge dryer. Consequently, it is thought that it would be prudent to not only monitor and regulate the temperature of the sludge being dried to avoid sludge combustion but also to monitor and regulate the temperature of the metal components of the sludge dryer during the drying process.
Thermal drying systems sometimes utilize different drying zones where the temperature and humidity within each zone is controlled at predetermined levels to regulate the rate of moisture content reduction of the material being dried. This concept is illustrated in U.S. Pat. No. 5,309,827 to Manser et al for a pasta dryer which illustrates the concept of different "climate" zones at various stages of the drying process. Other materials dryers which incorporate different drying chambers or zones to control the drying process include those illustrated in U.S. Pat. No. 4,472,887 to Avedian et al, U.S. Pat. No. 3,850,224 to Vidmar et al, and U.S. Pat. No. 2,981,528 to Culp. Still another dryer for controlling the moisture content of crumb rubber by monitoring the rubber temperature and the dryer air temperature at different zones in the dryer as a means for regulating the zone air temperature was disclosed in U.S. Pat. No. 3,367,038 to Bishop. Sr.
None of these referenced dryers are directed toward a dryer and a dryer control apparatus that will monitor and regulate not only the temperature of the material being dried but also monitor and regulate the temperature of the dryer itself. Such a monitoring system would allow the material drying efficiency of the dryer to be maximized and at the same time reduce the risk of occurrence of combustion and damage to the dryer and its components due to temperature induced expansion and fatigue.